This invention relates to an optical interconnection apparatus that is capable of optical switching and/or multiplexing/demultiplexing functions.
Fiber optic communication is becoming ubiquitous. Advantages to using fiber optic communications are well known and each year more users require increasing bandwidth to transmit ever-increasing amounts of information. Unfortunately at present, the cost of installing, xe2x80x9claying,xe2x80x9d new fiber is prohibitive. Hence, it would be greatly advantageous to be able to pass greater amounts of information through existing fiber optic networks. Wavelength division multiplexing is an effective method of exploiting the large bandwidth of optical fibers. In addition to increasing the transmission capacity of a point-to-point link, wavelength division multiplexing is also becoming important in optical networks for routing and circuit switching.
Fiber optic networks require optical wavelength multiplexers-demultiplexers. A multiplexer-demultiplexer is capable of functioning as a multiplexer or a demultiplexer. A demultiplexer separates a single multi-wavelength beam of light, a spectrum of light, into a plurality of beams each comprising a component wavelength of the multi-wavelength beam of light, and a multiplexer combines a plurality of light beams having different wavelengths into a single multi-wavelength beam of light.
An optical multiplexer-demultiplexer has a passband. The passband is a portion of light having wavelengths between first and second limiting wavelengths that are transmitted with minimum relative loss. An optical multiplexer-demultiplexer is designed to have a minimum attenuation, or alternatively stated a maximum transmittance, for a particular wavelength band. Notwithstanding intended design, multiplexer-demultiplexers generally do not have a constant transmittance. The transmittance of the device is wavelength dependent and periodic. Often, there is an undulated effect to the output response within the passband. Flattening of the output response within the passband of wavelength demultiplexers is desirable because it relaxes the requirements on the wavelength control of optical sources.
Some known multiplexers do not efficiently provide a flat output response within their passbands. U.S. Pat. No. 5,412,744 entitled xe2x80x9cFrequency Routing Device Having Wide and Substantially Flat Passbandxe2x80x9d by Corrado Dragone, issued May 2, 1995, discloses a frequency routing device in which a flat output response within the passband is achieved by combining a routing device with an optical coupler. This document and all references therein are herein incorporated by reference. One limitation of this device, however, is an inherent loss of optical power due to the presence of the coupler.
U.S. Pat. No. 5,488,680 entitled xe2x80x9cFrequency Routing Device Having Wide and Substantially Flat Passbandxe2x80x9d by Corrado Dragone, issued Jan. 30, 1996, discloses a frequency routing device in which a flat output response within the passband is achieved by coupling a first frequency routing device to a second frequency routing device. The output light from the first frequency routing device is launched into a second frequency routing device having a wavelength channel spacing equal to the free spectral range of the first device to provide a substantially flat output response within the passband. A ripple in the substantially flat output response is generally undesirable, though present in the above mentioned devices. In addition, the solution disclosed in U.S. Pat. No. 5,488,680 does not lend itself to bulk optics.
Precise synchronism between the spectral responses of the first frequency routing device and the second frequency routing device as disclosed in U.S. Pat. No. 5,488,680 is required for effective implementation of the technique. It would be advantageous if it was possible to tune the spectral responses of the first frequency routing device and the second frequency routing device, which are cascaded together, independently. In the case where the two cascaded frequency routing devices are present on a same integrated chip, it would be difficult to tune the spectral responses of the two devices independently after fabrication. It is also noteworthy that in frequency routing devices such as Arrayed Waveguide Grating (AWG) devices employed by Dragone, a fraction of power routed through the devices is diffracted into higher orders resulting in losses. Moreover, it is necessary to reduce crosstalk by blocking the optical power diffracted into the higher orders in a first Arrayed Waveguide Grating from entering a second Arrayed Waveguide Grating, as disclosed in an article in Technical digest Tuesday, Feb. 24, 1998, page 77, by Thompson, G. H. B. et al. For the aforementioned reasons implementation of the device of the devices disclosed in U.S. Pat. No. 5,488,680 are complex.
In addition, manufacture of integrated devices as disclosed in U.S. Pat. No. 5,488,680 is difficult because the integrated devices are lengthy and intricate. It would be advantageous to reduce ripple in the output response within the passband. It would also be advantageous to have a device that is manufactured economically, provides a substantially flat output response having reduced ripple over prior art devices within its passband. It would also be advantageous to reduce higher orders within the output response and to provide tunability of the device.
In accordance with the invention there is provided a wavelength multiplexer-demultiplexer comprising: a first routing device comprising an optical splitter having a first input port for launching a multi-wavelength beam of light having at least n wavelengths, wherein n is greater than 1, and a tapered MMI having a first face and a second opposing face shorter than the first face forming a taper therebetween, the second opposing face comprising a first output port for exiting sub-beams generated by the first routing device from the multi-wavelength beam of light, and a plurality of waveguides optically coupling the optical splitter to a first face of the tapered MMI.
The first routing device has a free spectral range that is approximately equal to the wavelength channel spacing of the second routing device.
In accordance with the invention there is further provided a wavelength multiplexer-demultiplexer comprising: a first routing device comprising an optical power splitter having a first input port for launching a multi-wavelength beam of light having at least n wavelengths, wherein n is greater than 1, and a MMI having a first output port for exiting sub-beams generated by the first routing device from the multi-wavelength beam of light, and a first plurality of waveguides optically coupling the optical splitter to the MMI; and, a second routing device comprising a first free-space region having a second input port; and a second plurality of waveguides of differing lengths forming an optical coupling between the first free-space region and a second free space region having a plurality of output ports, wherein the MMI is integral to the free-space region such that a boundary between the first MMI and the free-space region define the first output port of the first routing device and the second input port of the second routing device, wherein the first routing device has an output response with a free spectral range and the second routing device has a spectral response with a channel spacing which is approximately equal to the free spectral range provided by the first device.
In accordance with the invention there is provided a multimode interference coupler having a first boundary and a second opposing boundary shorter than the first boundary forming a taper therebetween, the first boundary comprising a first port optically coupled to a first waveguide and the second boundary comprising a second port.
In accordance with the invention there is also provided a multimode interference coupler having a first boundary and a second opposing boundary shorter than the first boundary forming a taper therebetween, the first boundary comprising a first port and the second boundary optically coupled to a second multimode interference coupler having a port, wherein the port of the second multimode interference coupler is a boundary having a common delimitation with the second boundary.